Conventionally, an image forming apparatus that forms an image by an electro-photographic system is provided with a fixing device for thermally fixing a toner image formed on a sheet of paper. Such a fixing device is known to include a belt heated by a heater, a nip member disposed inside a loop of the belt, and a pressure roller that forms a nip by nipping the belt between itself and the nip member, as disclosed in Japanese Patent Application Publication No. H05-27625, for example.